FIG. 1 shows a circuit diagram for a conventional voltage to current generator circuit 10. A differential amplifier circuit 12 has a non-inverting input (+) that receives an input voltage Vin. The inverting input of the differential amplifier circuit 12 receives a feedback voltage Vfb. The output of the differential amplifier circuit 12 drives the control terminal (gate) of an n-channel transistor 14. A first conduction terminal (source) of the transistor 14 is connected in a feedback path 16 to the inverting input of the differential amplifier circuit 12. A resistor 18 is coupled between the source of transistor 14 and a ground node. A second conduction terminal (drain) of the transistor 14 is coupled to an intermediate node 20. A current mirror circuit 22 includes a first p-channel transistor 24 and a second p-channel transistor 26. The first p-channel transistor 24 has a first conduction terminal (source) coupled to a supply voltage node Vdd. The second conduction terminal (drain) of transistor 24 is coupled to the intermediate node 20 and further connected to the control terminal (gate) of the transistor 24. Transistor 24 is accordingly a diode-connected device. The second p-channel transistor 26 has a first conduction terminal (source) coupled to the supply voltage node Vdd. A control terminal (gate) of the transistor 26 is coupled to the gate of the transistor 24. The second conduction terminal (drain) of transistor 26 outputs a current Iout having a magnitude that is dependent on the input voltage Vin.
The current through transistor 14 is sensed by the resistor 18 to generate the feedback voltage Vfb. The output of the differential amplifier 12 is the difference between the input voltage Vi and the feedback voltage Vfb. This difference voltage Vdiff is applied to the control terminal of transistor 14. In response to the negative feedback through feedback path 16, the differential amplifier 12 will drive the transistor 14 so that the feedback voltage Vfb equals the input voltage Vi. The current through transistor 14 is mirrored by the current mirror circuit 22 to generate the current Iout. The magnitude of the current Iout is set as a function of the input voltage Vin, the resistance of resistor 18 and the mirror ratio of transistors 24 and 26.
There is an offset associated with the operation of the differential amplifier 12. This offset will affect magnitude of the current Iout and thus introduces an error in the voltage to current conversion operation. There is a need in the art to address the voltage offset in order to ensure accurate operation of the voltage to current converter. More generally, for any amplifier circuit that suffers from a voltage offset, there is a need in the art to correct for that offset. Embodiments herein address that need.